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The urinary bladder

The urinary bladder is a hollow elastic organ that functions as the body’s urine storage tank. Urine produced by the kidneys flows through the ureters to the urinary bladder, where is it stored before passing into the urethra and exiting the body. The urinary bladder plays an important role in delaying and controlling urination so that the average person only has to urinate several times each day instead of constantly leaking small amounts of urine.

The urinary bladder is roughly spherical in shape, although its shape and size vary among individuals and depends greatly upon the volume of urine that it contains. It is located in the pelvic cavity anterior to the rectum and superior to the reproductive organs of the pelvis. In females the urinary bladder is somewhat reduced in size and must share the limited space of the pelvic cavity with the uterus that rests superior and posterior to it. During pregnancy the uterus takes up significantly more space and severely limits the expansion of the urinary bladder.

Many tiny wrinkles, known as rugae, line the inner surface of the urinary bladder and allow it to stretch as it fills with urine. A pair of ureteral openings on the inferior end of the posterior wall of the urinary bladder allow urine from the left and right ureters to enter the hollow lumen. A small funnel forms at the inferior end of the urinary bladder leading into the urethra, the tube that carries urine out of the body during urination.

The urinary bladder is made of several distinct tissue layers:

- The innermost layer of the bladder is the mucosa layer that lines the hollow lumen. Unlike the mucosa of other hollow organs, the urinary bladder is lined with transitional epithelial tissue that is able to stretch significantly to accommodate large volumes of urine. The transitional epithelium also provides protection to the underlying tissues from acidic or alkaline urine.
- Surrounding the mucosal layer is the submucosa, a layer of connective tissue with blood vessels and nervous tissue that supports and controls the surrounding tissue layers.
- The visceral muscles of the muscularis layer surround the submucosa and provide the urinary bladder with its ability to expand and contract. The muscularis is commonly referred to as the detrusor muscle and contracts during urination to expel urine from the body. The muscularis also forms the internal urethral sphincter, a ring of muscle that surrounds the urethral opening and holds urine in the urinary bladde. During urination, the sphincter relaxes to allow urine to flow into the urethra.

The urinary bladder sits in a unique position inferior to the peritoneum, a membrane that lines most of the abdominopelvic cavity. Due to its position, the outermost layer of the superior urinary bladder is made of serous membrane continuous with the peritoneum. Serous membrane provides protection to the bladder from friction between organs in the abdominopelvic cavity. The surface of the lateral and inferior sides of the urinary bladder forms a layer of loose connective tissue known as the adventitia. The adventitia loosely connects the urinary bladder to the surrounding tissues of the pelvis.

The urinary bladder functions as a storage vessel for urine to delay the frequency of urination. It is one of the most elastic organs of the body and is able to increase its volume greatly to accommodate between 600 to 800 ml of urine at maximum capacity. Transitional epithelium, elastic fibers, and visceral muscle tissue in the walls of the urinary bladder contribute to its distensibility and elasticity, allowing it to easily stretch and return to its original size several times each day.

It also helps to expel urine from the body during urination by contraction of the detrusor muscle and the relaxation of the internal urethral sphincter. Another separate muscle, the external urethral sphincter, surrounds the urethra just inferior to the bladder and helps to control and delay urination through its contraction. The external urethral sphincter is a skeletal muscle and therefore allows for the voluntary control of the urination reflex.
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